Thin films that protect or enhance the performance of an underlying material are commonly used in many industrial processes. Applications range from food packaging, hard coatings on eyeglass lenses, and window glass, to protection of integrated circuits, display screens, and photovoltaic panels. Such thin films need to be dense, have excellent adhesion to the underlying layer and not crack or peel for the life of the product. In some cases the coating needs to be a hermetic seal, keeping out water vapor and oxygen, and in this case it must not have too many tiny pinhole leaks that would disrupt the function of even tiny areas of the underlying structure or device. Further, for many of the most high value applications such as OLED display screens or lighting panels or organic photovoltaic panels, where the barrier needs to be tightest, the under-layers must not be exposed during manufacturing to temperatures above a limit that may range from slightly over a hundred degrees Celsius in some cases to less than about seventy five degrees for some polymers.
Presently, adhesion of deposited coatings is achieved either by first putting down an intermediate layer of highly wettable polymer layers, or by subjecting the surface to an inert plasma. Where the application is very cost sensitive it may be too expensive to use such wettable polymers, and for many inexpensive plastics an inert gas treatment has been found not to be effective in promoting adequate adhesion of the hard coating. Therefore, a more effective and less expensive method of ensuring adequate adhesion of coatings is needed.
For deposition of dense hermetic barrier layers at such low temperatures, sputtering of target material onto the substrate is the most common method used. This technique works quite well at substrate temperatures less than or about 100° C. but it generates substantial heat and produces films that are often not as amorphous and effective as barriers. In some applications where the plastic or polymer substrate is thick or cannot be cooled effectively sputtering may not be acceptable due to heating of the substrate. Plasma enhanced CVD has been used very predominantly in applications where the limiting temperature for the substrate is above about 200° C., but has not been capable of providing commercially competitive rates of deposition of high quality dielectrics at substrate temperatures under 100° C. The approach of Savas et al (US 20110006040, 20110005682, and 20110005681) offers promise that plasma enhanced CVD can provide dense homogeneous barrier films at such temperatures and at reasonable cost.
There are a few new and very demanding, high value applications for the hermetic encapsulation processes. Among these are encapsulation of organic and CIGS photovoltaic devices (PV), and encapsulation of organic light emitting diode (OLED) devices for both lighting and displays. These applications all have the strong requirement that the encapsulation be highly transparent to visible light, and very low in moisture and oxygen penetration. Solar panels using thin film materials such as CIGS or organic polymer for photovoltaic conversion require encapsulation with transmission rates from 10−4 to 10−5 gm/m2-day of water vapor. For these applications cost must be very low as well, the precise upper limit varying with the particular application. For applications such as OLED lighting and Organic PV modules the cost per square meter should be less than or about US$10/m2 since the total cost of such panels or web needs to be less than US$60/m2 and even as low as US$30/m2. For CIGS encapsulation cost must be no greater than $15/m2 to $20/m2, and for OLED displays encapsulation cost could be as much as $50/m2 since the display screen total cost of manufacture will be between about $1000/m2 to $2000/m2. Anti-reflective coatings that improve the light conversion efficiency of the PV devices are also in need and hard coatings as well for those PV devices that must be exposed to the outdoor environment. These must be very low in cost to be competitive in the energy market—typically less than US$3/m2 and in some other applications even less than US$1/m2.
One of the most demanding applications is OLED displays. Compared to commonly used liquid crystal displays (LCD), OLED technology can provide many benefits, including lower power consumption, higher contrast, wider viewing angles and the ability to be made on flexible substrates. But there are also substantial technological challenges to be solved before OLED displays larger than a square decimeter—such as useful for tablet or laptop displays—can be manufactured with high yield.
In particular, the very thin, low work-function metals used for the electron emitting layer in an OLED device are highly sensitive to damage by oxidation. Therefore, to achieve a useful lifetime in air, an OLED display must be encapsulated such that the oxygen transmission rate (OTR) is less than 10−3 to 10−5 scc/m2-day and water vapor transmission rate (WVTR) is even less than 10−7 g/m2-day. Currently this can only be done in mass production using a top covering of glass which is 100 or more microns thick. In comparison an LCD display is relatively insensitive to water or oxygen and requires encapsulation rated at OTR and WVTR of order 0.1 scc/m2-day or g/m2-day.
As a reference point to understand the needed tightness of such encapsulation, the air and moisture leakage requirement for an OLED display is equivalent to that of a high-vacuum chamber with a He leak rate on the order of 10−10 scc/sec. High-vacuum chambers with such a high degree of vacuum integrity are not uncommon, but require careful design, are expensive to make and are not generally mass-produced.
It is has been demonstrated that OLED displays can be sufficiently encapsulated when built on glass substrates by installing a top glass layer with a perimeter seal to the OLED area. As this perimeter seal is based on polymers it allows for some permeation or leakage of oxygen and water, requiring a “gettering” material in the space surrounding the OLED to absorb oxygen and water. This is an expensive technique ($50/m2 to $100/m2) and only suitable for relatively small and rigid displays, such as on smart phones or tablets. It also suffers from difficulty relating to stresses when front and back surfaces are not maintained at precisely the same temperature.
To reduce cost, increase manufacturing yield and increase applications for OLED both in lighting and displays there is a need to find methods and tools that enable high-volume production encapsulation with transparent thin films between about 30 nm and about 10 μm thick that provide the equivalent integrity of a high-vacuum chamber. In the case of flexible displays, which would be useful for many commercial applications, hard, inorganic barriers usually need to be less than about 100 nm thick to avoid cracking when the screen is flexed or the ambient temperature changes by several tens of degrees Celsius. The consequent leaking of atmosphere into the sensitive material layers destroys the device and makes a “black spot” on the screen or lighting panel. Other barrier materials may be mixed organic-inorganics that have both high transparency for visible light and barrier function while being up to 10 microns thick.
The prior art demonstrates that thin barrier films exist that have the ability to meet the requirements of OLED encapsulation under ideal conditions . Films consisting of inorganic nitrides, oxides, and oxynitrides are particularly suitable as they also are transparent. In particular, aluminum oxide, silicon nitride and silicon oxynitride are commonly used. These are highly transparent and yet very dense dielectrics.
However, under mass production conditions deposited films will have localized areas where the film does not have adequate barrier function. Such defects result from undesirable film morphology or insufficient film density that leads to localized areas that have higher transmission of oxygen and water vapor. Such defects may be caused by particles on the starting substrate, areas on the substrate with higher nucleation energy, particles generated during the deposition, overhanging or re-entrant surface topography and film cracks due to poor adhesion or stress.
Some prior art overcomes the effect of localized particles or other defects in one barrier layer by using multiple deposition layers, often with a planarizing, organic inter-layer between the inorganic barrier layers. In the case of the organic inter-layers the motivation is to bury the defects in the polymer and deposit each new barrier layer on the clean new surface. This causes a wide lateral separation of the defects in successive barrier layers such that the effective path length for transmission of oxygen and water molecules is substantially increased. The prior art suggest that as many as 3 to 7 repeated stacks of interlayer and barrier (films) are required to achieve oxygen or water vapor transmission rates (OTR & WVTR respectively) adequate for extended lifetime (up to 10 years) of an OLED display.
Another challenge to the application of thin barrier films to polymeric substrates in particular (and other surfaces in general) is the difficulty of obtaining adequate adhesion of the coating to the underlying material. This is particularly true of the high optical quality lower-cost polymers (such as acrylics or acrylates) used in both PV and displays. Typical approaches to improving adhesion include use of various plasma pretreatments (at low or atmospheric pressure) using oxygen, nitrogen, ammonia, as well as inert gases. Additional adhesion enhancement methods include deposition of thin metallic ‘primer’ layers using evaporation or sputtering, but these approaches can compromise the optical quality and operation of the display.
It should be noted that the desired Oxygen Transmission Rate (OTR) and Water Vapor Transmission Rate (WVTR) levels are well below the detection limit of about 10−3 g/m2-day for current commercial methods such as “MOCON”. Some services are able to measure levels about an order of magnitude lower, but are not able to distinguish between moisture leakage due to localized defects or due to bulk permeability. There are also more sensitive methods to determine leakage using a very thin layer of easily oxidized metal, such as Ca whose oxide is transparent. In this case localized defect areas can be seen as transparent areas. Finished OLED panels can also be tested, both for initial defects, and for lifetime.
Panels, modules or sheets of organic PV or CIGS are more cost sensitive than OLED applications and therefore cost effective thin film encapsulation can be an even more important enabler of the cost reductions that are essential for their competitiveness in the energy conversion marketplace. Currently, the cost of making PV panels is roughly US$1 per Watt so that their cost is roughly $100 to $150 per square meter. The encapsulation cost should therefore be no more than about 10% to 15% of this and yet must last for at least 5 years and probably more than 20 years. Since the panels produce the most electricity when exposed directly to sunlight it is likely that most of these must be able to withstand exposure to the elements and dust, and large ranges of temperature (−10° C. to 80° C.). When such panels use plastic substrate, which is far cheaper than metal or glass, they tend to have large thermal expansion coefficients—from about 20 microparts/degree Celsius to more than 100 microparts/degree Celsius. It is also essential that the encapsulation be able to stretch to accommodate the substantial thermal expansion of the substrate. While very thin (<30 nm) inorganic films such as silicon dioxide and silicon nitride accommodate substantial expansion without cracking, thicker films do crack. Since the efficiency of such panels is critical to their cost-effectiveness they would also benefit strongly by having anti-reflection coatings that could make them more efficient by reducing reflected light. Further, it would be helpful if such antireflection coating had an ability to resist scratching so that cleaning dust would not reduce the light transmission and efficiency. Such cleaning must be done several times a year to avoid efficiency loss. The sum of costs for all the above different coatings beneficial to the PV function should stay within the cost limits roughly of US$15 to US$20 per square meter. There are currently no known deposition processes that can produce said coatings within the cost limits. Were such a process found it would give an enormous boost to OPV and CIGS and to PV technology in general.
The above techniques do not allow for a method to monitor defect levels in production directly, and in the case of final test may not catch the effect of a long transmission path due to distributed defects in multiple layer stacks. The end result may be a display that works well initially, but may fail in a year of two, creating a negative perception in the market place.
There is, therefore, for OLED and possibly some PV technologies a need to develop deposition methods for thin films that have suitable bulk properties for low oxygen and water transmission rates. It is further necessary that such methods ensure excellent adhesion of encapsulation to the underlayers and avoid formation of local defects due to imperfections in the starting surface. It is further necessary that the number of defects per square meter be of order 1.0 and that the cost of this process not much exceed $10 per square meter, both in a mass production factory.